I have Bajaj Dominar 400 CC single cylinder bike with FI system.its 7k on odo now.Recently i pluged Diagnostic tool and found my LTFT is negative 11%.My bike has underbelly exhaust with catcon as slipon. Few days back i got to remove my catcon, i kept underbelly silencer with baffle as it is and i found my bike performance increased drasticaly, and when checked and i found LTFT started decreasing from -11 to - 8 %.

Does this indicates my company given stock exhaust system has more then required exhaust restriction? which is causing richer exhaust ?

STFT is variyng at idle and different RPM but almost near to zero.

Please advice.


Well as you have removed the catcon, the restriction and treatment to exhaust air has been removed which exits from the engine. The pressure drops around the catalytic converter is usually 50-200 Pa depending upon the engine RPM.

Thus when you removed the catalytic converter oxygen share in the increased which told the ECU that the air is rich in oxygen thus it has to lower the LTFT from 11% to 8 % which is a 3% increase in fuel or 3% less trimming of fuel.

LTFT decreased due to huge change in STFT as you removed the catalytic converter. Once LTFT stabilized by dropping to 8%, STFT is now stable at ~0% due to less changes needed every time.

Thus as suspected the stock exhaust does not put extra restrictions instead it gives the optimized exhaust to maintain the stochiometric point. While removing the cat converter it altered the original ratio. Plus as per motor vehicle to be legal it is necessary to have catcon which purifies the air.

You can check the value of pollution metrics in the next pollution check exam. The particulate matter concentration must have increased.

You can read more about LTFT and STFT here.

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